1. FIELD OF THE INVENTION
The present invention relates to a magnetic head and more particularly a magnetic head for writing and reading out data into and/or from a magnetic tape in magnetic recording apparatuses such as video tape recorders, audio tape recorders and the like.
2. DESCRIPTION OF THE PRIOR ART
A prior art magnetic head similar to the magnetic head disclosed in U.S. Pat. No. 4,485,420 or E.P. 0120518B1 is shown in FIGS. 1, 2 and 3. FIG. 1 illustrates a cross-sectional view, a part of which is omitted, taken along the longitudinal direction of travel of the magnetic tape over the magnetic head; FIG. 2 is a top view thereof; and FIG. 3 is a side view thereof. Reference numeral 1 is a case which is one of the component parts of the magnetic head and is made of a magnetic shielding material. A gap 3 is formed in a portion of a core 4 which contacts the magnetic tape. The core 4 is embedded in the case 1 by a resin 7. Tape guides 6 and 6' are provided on both sides of the case 1, respectively, in the longitudinal direction of travel of the magnetic tape.
The magnetic tape 5 is traveling over the upper surface of the magnetic head by which data is written into and/or read out from the magnetic tape 5. Concave portions 8 and 8' are formed at a portion of the upper surface of the magnetic head to which the resin 7 is exposed to prevent the magnetic tape 5 contacting the resin 7, so that deboning of magnetic particles from the magnetic tape 5 is prevented. As a result, the tape travel surfaces defined on the upper surface of the magnetic head are represented by 2a, 2b and 2b'.
It is now assumed that the magnetic tape 5 is traveling in the direction indicated by the arrow A in FIG. 2. Then the magnetic tape 5 comes into contact with the inclined plane formed on a projection 6b of the tape guide 6 and is biased to the reference plane formed on a projection 6a. As best shown in FIG. 3, the width of the magnetic tape traveling passage defined between the inclined plane and the reference plane is narrower than the width of the magnetic tape 5 and tension is exerted on the magnetic tape 5. Therefore, in order to ensure smooth travel over the surface of the magnetic head, the positions of the magnetic tape 5 in the direction of its thickness must be limited so that a bottom plane 6c is provided with between the projection 6a and 6b. The reference plane is substantially perpendicular to the bottom plane (to be referred to as the position control plane) 6c so that one side edge of the magnetic tape 5 travel along the reference plane. The reference planes defined on the projections 6a and 6a' are substantially perpendicular to the direction in which the gap 3 is extended so that the position of travel and the longitudinal direction of the magnetic tape 5 can be stabilized. As a result, the so-called gap angle between the tape 5 and the gap 3 can be maintained constant so that the azimuth loss due to the difference in the gap angle between a writing mode and a reading mode can be reduced.
In general, the tape guides are formed by injection molding of a plastic material which is inexpensive, thereby reducing the production cost, but almost all the tape guides thus fabricated have rough surfaces. As a result, due to the contact of the magnetic tape 5 with the bottom planes 6c and 6c' of the tape guides 6 and 6', it occurs frequently that the magnetic material and the binder of the magnetic tape 5 are separated.
In the case of the magnetic head of the type described above, the radius of curvature of the tape travel surface defined by the travel surfaces 2a, 2b and 2b' is relatively small and furthermore the bottom planes 6c and 6c' of the tape guides are more extended than the tape travel surfaces 2b and 2b'. Therefore, the passage of tape travel is located on the line interconnecting the bottom planes 6c and 6c' of the tape guides 6 and 6' and the top of the travel surface 2a containing the gap 3. It follows therefore that the magnetic tape 5 is barely in contact with the tape travel surfaces 2b and 2b'. However, when the magnetic head is used for a long time under the above-described conditions, dust such as magnetic particles and binder particles of the magnetic tape 5 separated at the bottom plane 6c and 6c' of the tape guides 6 and 6' attaches to the magnetic tape 5, is transported to the vicinity of the gap 3 and adheres there. As a result, such dust adversely affects the predetermined distance between the gap 3 and the magnetic tape 5 so that it becomes one of the causes of degradation of the writing and reading characteristics of the magnetic head.
In general, the adhesion or accumulation of such dust tends to occur at high temperatures so that the magnetic head of the type described above cannot be used in stereophonic audio equipment mounted in a car whose temperature becomes relatively high.
FIG. 4, 5 and 6 illustrate another prior art magnetic head. FIG. 4 is a top view thereof; FIG. 5 is a front view thereof and FIG. 6 is a perspective view thereof. Like reference numerals are used to designate similar parts in both the magnetic heads illustrated in FIGS. 1-3 and 4-6 and the description of the component parts already made with reference to FIGS. 1-3 shall not be repeated. It should be noted here that in the second prior art magnetic head, the tape guides are not illustrated.
In the second prior art magnetic head, the length of the magnetic core 4 perpendicular to the longitudinal direction of magnetic tape 5 is designed to be shorter than the width of the magnetic tape 5. For instance, while the length of the magnetic core 4 is 3.72.+-.0.05 mm, the width of the magnetic tape 5 is 3.81-0.05 mm to 3.81+0 mm so that over the surface 2a the magnetic tape 5 and the resin 7 come into contact with each other by the designation width of 0.09 mm.
As described above, because of the contact between the magnetic head and the magnetic tape, dust such as magnetic particles and binder particles tend to be separated from the resin 7 so that the dust particles are transported by the magnetic tape 5 and accumulate at the tape travel surfaces 2b and 2b'. In the case of the second-mentioned prior art magnetic head, the accumulation or adhesion of such dust particles is concentrated at the portions of the tape travel surface 2b or 2b' corresponding to both side edges of the magnetic tape 5 so that a thin dust film is formed between the magnetic tape 5 and the travel surface 2b or 2b'. As the result, a degree of wear at the portion at which such dust film is formed on the tape travel surface 2b or 2b' is different from the wear at the portion corresponding to the center portion of the magnetic tape 5 at which no dust adhesion or accumulation occurs. In general, the tape travel surfaces 2b and 2b' across the whole width of the magnetic tape 5 should be worn uniformally, but in practice because of the nonuniform wear described above, there arises the problem that the position of travel of the magnetic tape 5 cannot be uniformly maintained.
The resin 7 becomes a source from which dust particles are produced and such dust particles thus produced tend to adhere to the resin 7 as opposed to the tape travel surfaces. As a result, however the area of contact between the magnetic film 5 and the resin 7 is small, for instance, when the dust particles adhere to the resin 7 in the tape travel surface, the dust particles grow from the first adhered dust particle as a nucleus. Sometimes, such growing of dust particles prevents the smooth travel of the magnetic tape.